In a known secondary battery 31 as a secondary battery such as a lithium ion battery or a nickel metal-hydride battery, as shown in FIG. 9, a bottomed cylindrical battery case 32 accommodates an electrode plate group 33 and an electrolyte as power generation elements, a opening of the battery case 32 is sealed with an assembled sealing plate 35 via an insulating gasket 34, a lead (not shown) of one polarity extending from one end of the electrode plate group 33 is connected to the assembled sealing plate 35, and another lead (not shown) of the opposite polarity extending from the other end of the electrode plate group 33 is connected to the inner bottom face of the battery case 32. Insulating plates 36a and 36b are arranged on both ends of the electrode plate group 33.
The assembled sealing plate 35 includes, as shown in FIG. 10, a filter 37 being formed with vent holes 38, an inner surface of which faces the inside of the battery case 32, a cap 40 formed with exhaust holes 41, a outer surface of which faces the outside in a state of being insulated from the filter 37 by a gasket 39, and a safety vent mechanism 42 accommodated in the space between the filter 37 and the cap 40, which breaks when the pressure in the battery case 32 exceeds a predetermined pressure to interrupt the current path between the electrode plate group 33 and the cap 40 and to communicate the vent holes 38 with the exhaust holes 41. The safety vent mechanism 42 is made up of a lower valve body 43 and an upper valve body 44. The outer peripheries of the lower and upper valve bodies 43 and 44 are held between the filter 37 and the cap 40 with the gasket 39 interposed therebetween to provide insulation. The center parts of the lower and upper valve bodies 43 and 44 are connected to each other, and a breakable part is formed around the center, which breaks when pressure exceeding the predetermined level is applied (see, for example, Patent Document 1).
Another known conventional technique for improving the shock resistance of batteries is to set a plastic insulating ring between the electrode plate group and the sealing plate such that the upper part of the ring makes contact with the bottom of the sealing plate while the lower part makes pressure contact with the electrode plate group to retain the electrode plate group securely in place to restrict deformation of the leads due to impact or vibration (see, for example, Patent Document 2).
Yet another known conventional battery incorporates a feature for ensuring safe escape of gas wherein a plurality of projections are formed around gas outlet holes in the lower face of the sealing plate so as to prevent the holes from being blocked by the strip-shaped lead that connects the electrode plate group with the sealing plate even when the lead is pushed up towards the sealing plate due to a rapid build-up of gas pressure in the battery case (see, for example, Patent Document 3).
[Patent Document 1] Japanese Patent Laid-Open Publication No. 9-129195
[Patent Document 2] Japanese Patent Laid-Open Publication No. 2000-182592
[Patent Document 3] Japanese Patent No. 3596918